fix(process-registry): detach stdin from background subprocesses to prevent keyboard freeze

Background process non-PTY path used stdin=subprocess.PIPE unconditionally,
creating an orphan pipe that was never written to and never closed. Child
processes that read stdin would block indefinitely, competing with the
parent's prompt_toolkit event loop for terminal ownership and causing
complete keyboard lockout.

Change to stdin=subprocess.DEVNULL so children get immediate EOF on stdin
reads instead of blocking forever. For interactive stdin, the PTY path
(which has its own independent PTY via ptyprocess.PtyProcess.spawn) should
be used instead.

Fixes #17959

tools/process_registry.py

@@ -555,7 +555,7 @@ class ProcessRegistry:
             errors="replace",
             stdout=subprocess.PIPE,
             stderr=subprocess.STDOUT,
-            stdin=subprocess.PIPE,
+            stdin=subprocess.DEVNULL,
             preexec_fn=None if _IS_WINDOWS else os.setsid,
             creationflags=subprocess.CREATE_NO_WINDOW if _IS_WINDOWS else 0,
         )